PROJECT SUMMARY Our laboratory recently demonstrated that the lineage-restricted BROMO and WD40 domain containing epigenetic reader BRWD1 opens Ig? and enables assembly of RAG proteins at J?. I now report that BRWD1 has a much broader role in late B cell development. In the bone marrow, BRWD1 reshaped chromatin landscape by closing enhancers of genes expressed early in B cell development and opening those of genes expressed in late stages. In the absence of BRWD1, over 7000 genes were aberrantly expressed. While many of these genes are normally expressed in earlier developmental stages, many are also part of the germinal center (GC) transcription program. BRWD1 is first expressed in small pre-B cells. However, Brwd1 expression was much higher in nave follicular (FO) and highest in day 12 GC B cells. These data suggest BRWD1 could also regulate GC B cell programs. My data in WT GC B cells highly supports this. By using a novel 3- population gating strategy to isolate dark zone (DZ), light zone (LZ), and newly identified population grey zone (GZ), I found large transcriptional and genome accessibility differences between DZ and LZ cells. The GZ is also unique from DZ and LZ B cells based on transcription and genome accessibility, suggesting it may contain a novel GC B cell subpopulation(s). In LZ cells, BRWD1 is expressed in Myc- cells, a transcription factor repressed by BRWD1. Finally, analysis of genes differentially expressed in GC DZ and LZ, compared to those differentially regulated by BRWD1 in small pre-B cells, predicted that BRWD1 represses the DZ program and induces the LZ program. This has immunological implications as immunized mice lacking BRWD1 in GC B cells exhibited disorganized GCs and severely impaired affinity maturation. Overall, these data suggest similarities between genetic programs that separate proliferation and Ig? recombination in development and those that segregate proliferation from selection in GCs. Based on these findings, I hypothesize that by regulating enhancer accessibility BRWD1 is important to regulate transcriptional programs in novel GC B cell subsets. Additionally, I hypothesize that by repressing DZ and inducing LZ genetic programs, BRWD1 is critical for GC-dependent humoral immunity. These hypotheses will be tested in the following Specific Aims: Aim 1. Define transcriptional regulation of normal germinal center responses. Aim 2. Determine the role of BRWD1 in adaptive immunity.